A phase-locked loop (PLL) is a control system that generates an output signal having a phase and frequency derivative that is locked in fixed relation to an input signal. A PLL commonly includes a phase error detector, a low-pass loop filter, and a voltage-controlled oscillator (VCO). An input of the loop filter is coupled to an output of the phase error detector. An input of the VCO is coupled to an output of the loop filter. A first input of the phase error detector receives the input signal. A second input of the phase error detector is coupled to the output of the VCO to feed the output signal back to the phase error detector. A PLL may include any combination of analog and digital circuitry. An all-digital PLL (ADPLL) may include a numerically controlled oscillator (NCO) rather than a VCO, a digital loop filter, and an exclusive-OR phase error detector.
PLLs are commonly included in oscillator circuitry, among other types of circuitry. For example, a PLL may be used in a communications receiver circuit that recovers a clock signal from a received signal carrying both clock and data information.
The term “phase noise” refers to frequency-domain measurement of fluctuations in the phase of a signal caused by time-domain instabilities of the type commonly referred to as “jitter.” The term “close-in phase noise” refers to phase noise at a low frequency offset from the carrier frequency and outside the 1/f “flicker” noise, such as, for example, between 1 Hz and 1 kHz from the carrier frequency. In many types of circuitry, close-in phase noise does not present a problem. For example, common digital communications circuitry, such as a synchronous optical networking (SONET) receiver, is sensitive to phase noise (or jitter) in the 12 kHz to 20 MHz range. For this reason, manufacturers of oscillator circuits commonly sacrifice close-in phase noise to obtain low phase noise in the 12 kHz to 20 MHz range.
Some digital communications technologies, such as the Digital Subscriber Line (DSL), operate in frequency ranges above about 50 MHz. For example, although DSL circuitry may employ any of a number of reference frequencies, one commonly employed DSL reference frequency is 70.656 MHz. Some of these communications technologies, including DSL, rely on accurate analog-to-digital conversion. To provide such analog-to-digital conversion, DSL circuitry may require high frequency (e.g., greater than 50 MHz) PLL-based oscillator circuitry with low close-in phase noise. More specifically, very accurate clock signals, i.e., having good signal to noise ratio, at frequencies close to the carrier or center frequency, are required to recover the data. However, commercially available oscillator circuits that are configurable to operate in a specified frequency band (e.g., DSL) commonly have undesirably high close-in phase noise.
Crystal oscillators capable of generating high frequencies (greater than, for example, 50 MHz) are known as third-overtone crystal oscillators because they resonate at three times their fundamental frequencies. However, third-overtone crystal oscillators suffer from poor close-in phase noise performance. Fundamental-mode crystal oscillators, which resonate at their fundamental frequency, have better close-in phase noise performance than third-overtone crystal oscillators. However, fundamental-mode crystal oscillators are generally not capable of generating high frequencies (greater than, for example, 50 MHz). (This is because resonant frequency is inversely proportional to crystal thickness, and present manufacturing processes cannot handle extremely thin crystals.) A further consideration is that fundamental-mode crystal oscillators are generally not commercially available in, for example, the specific reference frequencies required (e.g., the above-referenced 70.656 MHz DSL reference frequency). Rather, such oscillators are commercially available in a small number of generic frequencies, with the understanding that the oscillator output signal can be multiplied (or divided) in frequency if desired. However, multiplying the output frequency of an oscillator can increase close-in phase noise.